In the past, in image reading devices used for image forming apparatuses, such as copy machines, printers, and facsimile devices, original documents are sequentially fed from a document placing section by a document conveyance section.
When a xenon lamp is used as an illuminating lamp to illuminate an original document, the amount of light is initially decreased during continuous lighting due to its self generated heating, after which the amount of light emitted from the xenon lamp becomes stable. Said decrease of the amount of light can result in reading errors, such as non-reading of lines structuring a letter, so that continuous correction is necessary to keep stable light amount.
In the conventional art, a standard white plate is mounted adjacent to the image reading section, as a means for said correction. The amount of light, emitted from a light source and reflected by the standard white plate, is measured by an image pickup device, carrying an image reading function and a light amount measuring function, whereby said measured light amount is compared with a reflected light reference value which was previously obtained by the experiment. If any difference exists between them, the amount of light, measured by the image pickup device, is corrected to be equal to the standard reflected light amount by a control section, which uses a shading correction method.
Further, in the shading correction during image reading, various methods for mounting a correcting member, coated with a standard color for the standard white plate, are provided. Unexamined Japanese Patent Application Publication No. 5-14,717 discloses a rotatable roller whose outer circumferential surface has a portion coated with standard white color and a portion coated with standard black color, and said roller is mounted to face the image pickup device, whereby either portion can be selected.
Still further, Unexamined Japanese Patent Application Publication No. 2003-134,301 discloses a rotatable and white roller, serving as a guide member, which faces the image pickup device and is separated from the image pickup device at a predetermined distance. Said guide member includes a home position surface for the stand-by of the apparatus, an original document guide surface for reading the original document, and a shading correction surface for the shading correction.
However, in both Patent Applications, each member coated with the standard color is a roller. Since the roller has a curved surface to face the image pickup device, if an image reading line shifts slightly toward a document conveyance direction, the amount of light reflected by the standard white color after having been emitted from the light source, tends to vary, which tends to result in a less-accurate shading correction.